The Ecology of Herbivore-induced Silicon Defences in Grasses
S.E. Hartley*1 and J.L. DeGabriel2
1York Environmental Sustainability Institute, Department of Biology, University of York,
Heslington, York, YO10 5DD, UK.
2Hawkesbury Institute for the Environment, Western Sydney University,
Locked Bag 1797, Penrith, NSW 2751, Australia.
*Corresponding author: [email protected]
Summary
1. Silicon as a defence against herbivory in grasses has gained increasing recognition and has
now been studied in a wide range of species, at scales from individual plants in pots to plant
communities in the field. The impacts of these defences have been assessed on herbivores
ranging from insects to rodents to ungulates. Here we review current knowledge of silicon
mediation of plant-herbivore interactions in an ecological context.
2. The production of silicon defences by grasses is affected by both abiotic and biotic factors
and by their interactions. Climate, soil type and water availability all influence levels of
silicon uptake, as does plant phenology and previous herbivory. The type of defoliation
matters and artificial clipping does not appear to have the same impact on silicon defence
induction as herbivory which includes the presence of saliva. Induction of silicon defences
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
has been demonstrated to require a threshold level of damage, both in the lab and the field. In
recent studies of vole-plant interactions, the patterns of induction were found to be
quantitatively similar in glasshouse compared with field experiments, in terms of both the
threshold required for induction and timing of the induction response.
3. The impacts of silicon defences differ between different classes of herbivore, possibly
reflecting differences in body size, feeding behaviour and digestive physiology. General
patterns are hard to discern however, and a greater number of studies on wild mammalian
herbivores are required to elucidate these, particularly with an inclusion of major groups, for
which there are currently no data, one such example being marsupials.
4. We highlight new research areas to address what still remains unclear about the role of
silicon as a plant defence, particularly in relation to plant-herbivore interactions in the field,
where the effects of grazing on defence induction are harder to measure. We discuss the
obstacles inherent in scaling up laboratory work to landscape-scale studies, the most
ecologically relevant but most difficult to carry out, which is the next challenge in silicon
ecology.
Key-words: defence induction, insect, physical defences, silica, plant-herbivore
interactions, herbivory, landscape-scale, mammal.
Introduction
Silicon is the second most abundant element in the Earth’s crust and, in grasses at least, may
be present in greater amounts than macro-nutrients, comprising up to 10% dry weight in
2
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
some species (Epstein 1999). Several hypotheses for an ecological role for this extensive
accumulation have been put forward over recent years (Raven 1983; Ma 2004; Massey,
Ennos & Hartley 2007a; Cooke & Leishman 2011), with one of the earliest suggestions being
that silicon was a defence against herbivory. In agricultural systems, it has long been known
that silicon enhances the resistance of crop plants to insect pests (e.g. McColloch & Salmon
1923; Ponnaiya 1951; Sasamoto 1953; Keeping, Meyer & Sewpersad 2013) and that
application of soluble silicon leads to decreased damage by insect herbivores (Goussain,
Prado & Moraes 2005). The effects of silicon augmentation on crop-pest interactions has
been the subject of previous reviews (Keeping & Reynolds 2009; Reynolds, Keeping &
Meyer 2009); here we focus specifically on ecological systems and on the biotic and abiotic
factors which affect the natural induction of silicon-based defences.
In one of the first studies in natural ecosystems, McNaughton and Tarrants (1983) proposed
grass leaf silicification as an “inducible defence” against vertebrate herbivores following their
findings that grasses from grazed areas in African savannas had higher silicon contents than
those from ungrazed ones, and that clipped plants accumulated more silicon than undamaged
ones. However, some grasses had intrinsically higher silicon contents, even when ungrazed,
so the authors concluded silicon was “best viewed as a qualitatively constitutive trait that is,
nevertheless, quantitatively inducible by grazing” (McNaughton & Tarrants 1983). This
work, supported by other early ecological studies (e.g. McNaughton et al. 1985; Brizuela,
Detling & Cid 1986; Cid et al. 1990) suggested that silicon provided wild grasses with an
effective defence against herbivores that could be rapidly mobilised in response to attack
(Karban & Baldwin 1997), contrasting with previous notions that grasses were relatively
undefended (Vicari & Bazely 1993).
3
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
Silicon defences are usually deployed as phytoliths or other forms of amorphous silica (SiO2)
in the leaf epidermis, or deposited in spines, trichomes or hairs on the leaf surface (Currie &
Perry 2007; Hartley et al. 2015; Strömberg, Di Stilio & Song 2016). These structures render
leaves tough and abrasive and therefore physically deter herbivores from feeding (Massey &
Hartley 2006; Massey & Hartley 2009). In addition, they have been shown to reduce the
digestibility of grasses (Shewmaker et al. 1989), act as a structural inhibitor of microbial
digestion in ruminants (Harbers & Thouvenelle 1980; Harbers, Raiten & Paulsen 1981) and
stimulate other plant defence mechanisms (Goussain, Prado & Moraes 2005; Fauteux et al.
2006; Ye et al. 2013). Adverse effects of silicon on rates of herbivory and animal
performance have now been demonstrated on a range of insect herbivores (Massey, Ennos &
Hartley 2006; Massey & Hartley 2009; Reynolds, Keeping & Meyer 2009; Keeping, Miles &
Sewpersad 2014), rodents and lagomorphs (Gali-Muhtasib, Smith & Higgins 1992; Massey
& Hartley 2006; Cotterill et al. 2007; Huitu et al. 2014; Wieczorek et al. 2015a; Wieczorek et
al. 2015b) and ruminants (Massey et al. 2009). Studies on wild mammalian herbivores
remain relatively lacking however, in marked contrast to the numbers of studies on the effects
of silicon on agricultural insect pests (Massey, Ennos & Hartley 2006; Kvedaras et al. 2009;
Reynolds, Keeping & Meyer 2009; Keeping, Miles & Sewpersad 2014).
More recent work has expanded our understanding of silicon induction, i.e. the increase in
silicon accumulation that occurs in plants when they are damaged, and its similarities and
contrasts with other inducible defences. In common with many types of inducible plant
defences, induction of silicon is often greater in response to attack by herbivores than to
artificial clipping (e.g. Massey, Ennos & Hartley 2007b; Quigley & Anderson 2014),
although in contrast to other types of defence (Tanentzap, Vicari & Bazely 2014), the role of
herbivore saliva in the expression of silicon is unclear. It also appears to be non-linearly
4
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
related to both the frequency and intensity of damage, requiring multiple damage events and
a threshold amount of biomass to be removed (Massey, Ennos & Hartley 2007b; Reynolds et
al. 2012). It appears that the response of plant silicon levels to damage, particularly in the
case of clipping, varies with plant species, genotype and phenological stage, as well as
damage intensity (Kindomihou, Sinsin & Meerts 2006; Soininen et al. 2013). Unlike many
induced defences (but see Haukioja & Neuvonen 1985), silicon induction persists for several
months (Reynolds et al. 2012), reflecting the recalcitrant nature of silicon phytoliths, which
are not remobilised once formed (Piperno 2006; Strömberg, Di Stilio & Song 2016), and
hence tend to accumulate as leaf tissue ages. This persistence has consequences for the
impact of induced silicon defences on herbivores, particularly for small mammals where
delayed density-dependent effects drive population dynamics (Lindroth & Batzli 1986;
Ergon, Lambin & Stenseth 2001; Smith et al. 2006; Ergon et al. 2011). A time lag in defence
induction, due to the requirement for persistent herbivory and the long “decay time” of
induced silicon levels, could provide a mechanism for such delayed plant-herbivore
feedbacks (Massey et al. 2008). Despite many experimental demonstrations of the
importance of silicon in plant-herbivore interactions, there are cases where no changes in
plant silicon levels in response to herbivory are observed, as well as examples of herbivores
unaffected by silicon-based induced defences (e.g. Banuelos & Obeso 2000; Redmond &
Potter 2006; Damuth & Janis 2011).
Studies on silicon mediated plant-herbivore interactions now encompass a wide range of
natural grass species and include scales from individual plants in greenhouses to plant
communities in the field (Massey, Ennos & Hartley 2007b; Reynolds, Keeping & Meyer
2009; Soininen et al. 2013), allowing us to ask whether consistent patterns are emerging in its
5
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
accumulation and impact, as well as assess which aspects of silicon induction remain poorly
understood. We aim to address the following questions in this review:
(i) How do biotic (specifically herbivory) and abiotic factors influence the
production of silicon defences by natural grasses?
(ii) How does silicon uptake by these grasses impact on different classes of
vertebrates and invertebrate herbivores?
(iii) Do silicon defences provide a viable hypothesis for explaining population
regulation of wild grazing herbivores?
We review our current state of knowledge around these specific questions and summarise
gaps in our understanding of each of these questions. We also suggest possible approaches
for scaling up laboratory work to landscape-scale studies, an exciting future challenge in the
study of silicon-based defences that is essential for answering the third of these questions. We
focus on grasses as this plant family has been the most comprehensively studied in terms of
ecological aspects of silicon-mediated interactions between plants and their herbivores,
although there is evidence of silicon induction in other angiosperm groups (Hodson et al.
2005; Cooke & Leishman 2011; Katz 2015).
Impact of herbivory: silicon induction varies with the type, amount and
timing of damage
One of the features of silicon-based defences which has been frequently demonstrated is that
herbivory induces silicon accumulation to a greater extent than does artificial clipping (e.g.
Massey, Ennos & Hartley 2007b; Quigley & Anderson 2014). This is particularly the case in
studies of mammalian herbivores, with relatively few studies demonstrating this differential
effect in the case of insect herbivory (but see Gomes et al. 2005; Massey, Ennos & Hartley
2007b). For example, in North American studies, grasses from areas that had been heavily
6
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
grazed by prairie dogs showed elevated concentrations of silicon compared to more lightly
grazed ones, but mechanical defoliation did not induce this response, with silicon levels in
clipped leaves lower than those in unclipped ones (Brizuela, Detling & Cid 1986; Cid et al.
1989; Cid et al. 1990), whereas in other cases, clipping led to induction in some grass
species, but not in others (e.g. McNaughton et al. 1985; Kindomihou, Sinsin & Meerts 2006;
Quigley & Anderson 2014). A recent literature review demonstrated that silicon induction
was highly variable between species and dependent on the frequency and intensity of damage
(see below), but on average, induction was more than twice as great in response to herbivory
than to manual defoliation across 34 species/study combinations (Quigley & Anderson 2014).
Natural herbivory elicits a greater induction of defences than mechanical wounding, (e.g.
Hartley & Lawton 1987; Hartley & Lawton 1991; Valkama et al. 2005; Farmer 2014)
mediated through herbivore-specific molecular and physiological plant responses (e.g. Korth
& Dixon 1997; Reymond et al. 2000). Oral secretions provide herbivore-specific cues for
defence induction in many insects (Hartley & Lawton 1991; Alborn et al. 1997; Bonaventure,
VanDoorn & Baldwin 2011; Tian et al. 2012). Components of insect saliva, plant cell wall
fragments and other cues create a signalling cascade which triggers a defence response,
including the production of the so-called “wound hormones” (jasmonic acid (JA) and
salicylic acid), changes in gene expression and increases in secondary metabolites (Heil &
Ton 2008; Bonaventure, VanDoorn & Baldwin 2011; Stam et al. 2014). Equivalent research
on induced defence responses to vertebrate herbivory is relatively lacking (Walters 2010),
although, Tanentzap et al. (2014) recently provided a breakthrough by demonstrating that
moose and reindeer saliva could counter alkaloid defences produced as a result of a grass-
endophyte mutualism. In the case of silicon defences, there has not yet been any test of
7
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
whether the application of herbivore saliva induces uptake to the same extent as actual
herbivory.
Nevertheless, it is apparent that silicon addition can lead to increased expression of a large
spectrum of inducible defence responses and amplifies the JA-mediated induced defence
response by serving as a priming agent for the JA pathway, whilst JA promotes Si
accumulation (Fauteux et al. 2006; Ye et al. 2013). A better understanding of the
mechanisms underlying silicon induction, the impacts of silicon uptake on other defence
pathways in plants, and the reasons for any observed differences in induction in response to
clipping, insect and vertebrate herbivory would enable us to answer important questions
about the ecological role of silicon. For example, we may gain insights into whether silicon
defences can explain the higher levels of dietary specialisation among insect herbivores and
tight pairwise coevolution between insects and their host plants, which is generally less
common amongst mammals, particularly grazers.
There are other differences between clipping and herbivory relating to the various ways
herbivores feed. Lepidoptera usually feed by shearing off plant material with their incisors,
gramnivorous orthopterans rely on the molar regions of their mandibles to mechanically
disrupt the cell wall, whilst phloem-feeding insects such as aphids use a piercing and sucking
mechanism (Bonaventure 2012). Each of these actions is likely to damage plant cells in a
different way and to a greater extent than would mechanical snipping, which results in a
cleaner cut and less disruption to the plant cells, hence we might expect differences in the
effects of herbivory between different guilds of insects and mammalian herbivores.
8
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
In fact, we still have surprisingly little data on the relative magnitude of silicon induction by
different types of herbivore (but see Quigley & Anderson 2014). It is possible that herbivory
by some species of mammalian herbivores might not result in the induction of chemical or
physical defences, since the speed, pattern and amount of leaf removal might negate the
signal for the plant to respond (Walters 2010). Some small mammals, such as voles
selectively remove the basal meristems of grasses and may disrupt the cell walls, whereas
larger herbivores, such as ungulates, remove large portions of the above ground biomass in a
single bite, a very different type of tissue wounding. There are few studies addressing this,
though Massey et al. (2007b) compared silicon induction in response to mechanical damage
and herbivory by locusts and voles. They demonstrated that although both types of herbivory
induced silicon defences more than clipping, there was no difference between the impacts of
the two herbivores on two different natural grasses.
Despite the tendency for insect and mammalian herbivores to elicit induction of silicon
defences, this pattern is not universal; some studies have found that herbivory did not cause a
measureable induction of silicon defences (e.g. Soininen et al. 2013; Quigley & Anderson
2014). These examples tend to be field-based studies comparing silicon levels in grasses in
grazed and ungrazed areas, where the levels of herbivory are unknown and maybe of
insufficient duration and/or intensity to elicit induction (see below), and where other site-
based factors, e.g. local climate, soil type, or previous grazing history, may influence
induction (e.g. Georgiadis & McNaughton 1990; Fenner, Lee & Duncan 1993; Soininen et al.
2013). Laboratory studies may provide an explanation as some have demonstrated that
silicon induction may require a threshold of damage, either in terms of amount of biomass
removed or in terms of frequency of damage (Massey, Ennos & Hartley 2007b; Reynolds et
9
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
al. 2012). These studies suggest a single instance of damage does not lead to induction, nor
do damage levels of less than around 20% of total leaf area removed.
Case study: The effects of grazing by voles on silicon induction in the field
The complexity of the relationship between induction and damage intensity has been difficult
to resolve given the lack of studies in the field; clear thresholds of herbivore damage required
to induce elevated silica accumulation have only been demonstrated in laboratory studies.
Recently, we conducted field experiments using specially-constructed grazing enclosures
which exposed Deschampsia caespitosa plants to varying intensities of grazing by field voles
(Microtus agrestis) to test the effects of grazing intensity and season on silicon induction (J.
DeGabriel, S. Hartley, F. Massey, S. Reidinger and X. Lambin, unpublished data). We
compared our field results to the laboratory results of Reynolds et al. (2012), using the same
study system.
Methods
Experimental design
We erected a 36 m x 36 m grazing enclosure, consisting of 81 4 m x 4 m cells in an area of
natural clear cut grassland in Kielder forest in northern England that is habitat for populations
of field voles. The enclosures were constructed from vole-proof wire mesh, which was
sunken 30 cm below ground and was at least 50 cm high, topped with a roll-top, which
prevented voles from moving into neighbouring cells. The dominant plant species in each of
the experimental cells was D. caespitosa, which is a major dietary component of field voles
and their main overwinter food source. The enclosures were exposed to natural levels of vole
grazing in previous years, but we trapped and removed all voles from the enclosures in the
winter before commencing our experiment in spring.
10
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
From March 2009, we live-trapped wild voles in surrounding grassland using Ugglan traps
(Grahnab, Marieholm, Sweden) and immediately introduced a single vole into each of 12
cells (giving a density of 50 voles/ha) and 6 voles into each of another 12 cells (giving a
density of 300 voles/ha). The sex and body mass of each vole was recorded. Voles were
allowed to graze freely in the cells for 3-4 days, before we re-trapped and released them
outside the enclosures. We repeated this grazing treatment roughly every six-seven weeks
between March and November 2009, as well as in January, February and April 2010. Ability
to access field sites over winter was restricted due to heavy snow.
We collected samples from a single D. caespitosa tussock in each enclosure approximately
one month after each grazing treatment. Within each cell, we randomly chose 3 tussocks on
each sampling occasion and took 5 tillers each from the centre and edge of those tussocks.
We pooled the leaves from the three plants in plastic bags and stored them frozen at -20°C for
analysis. The leaves chosen were the youngest fully expanded and undamaged adult leaf
blades available that were green and not contaminated with fungus, which we considered to
be the most palatable to voles. Thus, at different times of year, the leaf samples were not
exactly the same, as we deliberately did not collect new or young leaves that had not fully
matured. We prepared and analysed the silicon content of the leaf samples using portable X-
Ray Fluorescence (Reidinger, Ramsey & Hartley 2012).
In September 2009, we estimated the average grazing damage levels on D. caespitosa. We
randomly selected a single tussock in each cell and haphazardly chose 100 leaves on the
outside of the plant (covering the entire circumference of the tussock) and 100 leaves on the
11
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
interior. We visually recorded how many of these leaves had been damaged by vole grazing
and averaged the proportion of leaves damaged across the plant.
Results
Effects of grazing intensity on silicon induction
We found that on average, approximately 5 % of leaves were damaged in the 50 voles/ha
treatment and 23.5 % of leaves were damaged in the 300 voles/ha treatment. This was
roughly equivalent to the “low” (5% of leaves removed) and “high” (20% of leaves removed)
grazing treatments imposed in the laboratory study by Reynolds et al. (2012). We found
remarkably similar patterns in the rates of silicon induction under the high and low grazing
pressures in the field (Figure 1a) to those reported by Reynolds et al. (2012). In both the lab
and the field, silicon induction only occurred under the high grazing intensity treatment, but
not the low. Furthermore, induction was delayed for two months after initiation of grazing,
before an approximate doubling of silicon concentrations in the high, relative to the low
grazing treatment by five months after the start of damage.
Effects of season on silicon induction
We found that silicon concentrations increased in D. caespitosa in response to vole grazing
during the summer and autumn, reaching a peak in winter, presumably as a result of
accumulation in old leaves from the previous growing season. Concentrations of silicon then
decreased rapidly in the spring, again presumably as a result of flushes of new leaves that had
not taken up silicon (Figure 1b).
Our results demonstrate that both threshold effects and seasonality are important in silicon
uptake, and these factors have been found to influence induction in other studies. For
12
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
example, in a study of Agrostis tenuis, Banuelos and Obeso (2000) found that silicon content
of plants was higher in heavily grazed areas than within experimental exclosures during the
summer, but no such differences were apparent in winter. This was in contrast to the results
from our experimental field enclosures in northern England (Figure 1b). There are similar
phenological effects in plant responses to clipping: in a study of 5 tropical grass species,
silicon content generally increased over time, although this effect varied with species, and for
some species the effect of clipping on leaf sheath silicon content differed between dates
(Kindomihou, Sinsin & Meerts 2006). Similarly, the effect of mowing on the silicon levels of
prairie foliage differed between July, when there was no effect, and October, when there was
an increase (Seastedt, Ramundo & Hayes 1989).
There is also evidence that phenological variation in silicon content may differ between grass
species growing in different locations. For example, in North American prairies shoot silicon
concentrations increase throughout the growing season (Brizuela, Detling & Cid 1986;
Seastedt, Ramundo & Hayes 1989), and the same trend was found in savanna grasses in
Kenya (Georgiadis & McNaughton 1990). In contrast, in another African study, grasses in the
Serengeti in Tanzania, had higher silicon levels early in the growing season (McNaughton et
al. 1985). This variation is more likely related to broader ecosystem differences across
latitudes than effects of season per se. Clearly induction of silicon defences, whether in
response to artificial damage or natural herbivory is highly variable and its magnitude is
contingent on a number of factors, including damage type, damage intensity, timing of
damage, plant species and even tissue age (see below, Banuelos & Obeso 2000; Kindomihou,
Sinsin & Meerts 2006).
13
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
Impact of herbivory: induction of silicon defences varies with plant species
and genotype
The ability of different non-agricultural grass species to increase their silicon uptake in
response to experimental removal of leaves by herbivores has been measured across a
relatively narrow range of species under controlled conditions (Massey, Ennos & Hartley
2006; Massey, Ennos & Hartley 2007b). In contrast, many studies have assessed such
variability in relation to clipping and have demonstrated clear between-species differences in
silicon uptake in response (e.g. McNaughton et al. 1985; Kindomihou, Sinsin & Meerts 2006;
Soininen et al. 2013). Between-species variation in the magnitude of the differences in silicon
levels in wild plants collected from naturally grazed versus ungrazed areas is also well-
documented (e.g. McNaughton & Tarrants 1983; Brizuela, Detling & Cid 1986; Soininen et
al. 2013). Such differences have also been demonstrated within-species, which has led to the
suggestion that herbivory drives the selection of ecotypes with increased ability to take up
silicon (Detling & Painter 1983; McNaughton & Tarrants 1983; Banuelos & Obeso 2000).
While that idea remains somewhat speculative for field populations, the existence of intra-
specific genotypic differences in silicon induction in response to clipping is clear in
laboratory experiments. For example, the silicon content of some genotypes of A. tenuis
increased after clipping, whereas it declined in others (Banuelos & Obeso 2000). Similarly,
Soininen et al. (2013) found that four different grass species showed marked within-species
differences in silicon content following clipping, in addition to extensive between-species
variation. Similarly, three grass species from the same genus, Festuca, showed very different
patterns of silicon uptake and deposition in defensive structures (spines and phytoliths) in
response to artificial damage and manipulation of silicon supply, as did two genotypes of one
of these species, F. arundinacea (Hartley et al. 2015).
14
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
We do not know why there are such large differences in silicon content in quite closely
related species and even between genotypes of the same species. This is because we have a
very limited understanding of silicon uptake at the physiological, biochemical and molecular
level for most non-crop species (Hartley et al. 2015; though see Deshmukh & Belanger
2016). In crop species, particularly rice, many of the transporters responsible for silicon
uptake and distribution within the plant have been identified and their role characterised (Ma
et al. 2006; Ma & Yamaji 2006; Ma 2009; Ma & Yamaji 2015; Yamaji et al. 2015), but we
still have limited understanding of how the impact of damage on silicon uptake and
deposition interacts with abiotic factors. In addition, grasses have an array of different types
of defences, which is a complicating factor with respect to disentangling silicon dynamics.
Thus far, few studies (but see below Quigley & Anderson 2014; Wieczorek et al. 2015b)
have attempted to simultaneously quantify experimentally the relative importance of biotic
factors, such as grazing or other grass defences, and abiotic factors, such as water availability,
on silicon uptake, particularly in the field.
Abiotic factors: induction of silicon defences in response to herbivory
varies with soil type, water availability, and climate
Abiotic factors influence silicon levels and can impact silicon defences (Soininen et al.
2013), although in many studies it is hard to disentangle abiotic from biotic influences,
particularly grazing levels. For example, it is unclear whether higher levels of silicon
observed in plants from grazed sites in the North American prairies (Brizuela, Detling & Cid
1986) or the Serengeti (McNaughton et al. 1985) are due to a direct response to herbivory
(i.e. induction), or to other abiotic differences between the sites. However, it is clear that
plants from more heavily grazed sites could accumulate more silicon in leaves than those
from ungrazed ones in the laboratory (Detling & Painter 1983; McNaughton & Tarrants
15
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
1983), suggesting some role for biotic drivers, regardless of abiotic conditions. However,
uptake ultimately depends on availability of silicon, itself dependent on soil type and soil pH
(Beckwith & Reeve 1964; Ehrlich et al. 2010) and, since silicon enters the plants in soluble
form through the transpiration stream, it may also depend on water availability and climatic
factors which influence transpiration, such as temperature (e.g. Raven 1983; Sangster,
Hodson & Tubb 2001; Kindomihou, Sinsin & Meerts 2006; Faisal et al. 2012). However, the
extent to which silicon uptake depends on transpiration rate remains a subject of debate
(Hartley et al. 2015).
A recent study by Wieczorek et al. (2015b) attempted to disentangle the relative importance
of abiotic and biotic factors in silicon accumulation in a natural wetland system, where the
dynamic hydrology might be predicted to have as large an impact as herbivory on the silicon
content of foliage. They demonstrated the importance of abiotic factors in silicon
accumulation in grazed systems, with temperature and snow cover influencing silicon uptake
in both leaves and rhizomes of a tussock sedge, whilst the level of winter flooding affected
uptake in the rhizomes, but not in the leaves. However, although both herbivory and abiotic
conditions influenced the uptake of soil available silicon by plants in this study, grazing
appeared to be a more important driver than hydrology for foliar tissue (Wieczorek et al.
2015b). This contrasts with the study by Quigley and Anderson (2014), which found water
availability had a greater impact on natural silicon levels than defoliation in one of the two
species tested, although this study used clipping rather than natural herbivory.
Abiotic and biotic factors may interact in determining both the levels of silicon-based
defences and their impact on herbivores. For example, the effectiveness of silicon-based plant
defences against locusts has been shown to differ between plant species according to soil
16
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
silicon availability. Under low soil silicon availability, the herbivores removed more leaf
biomass from L. perenne than from P. annua, whereas under high silicon availability the
reverse was true. Consequently, herbivory shifted the competitive balance between the two
grass species, with the outcome depending on the availability of soil silicon (Garbuzov,
Reidinger & Hartley 2011). Overall, we see evidence that abiotic factors influence silicon-
based responses to damage in plants, but we currently lack comprehensive experimental
evidence of these influences, particularly in the case of field studies involving herbivores.
Interactions between environmental drivers such as soil silicon and water availability and
induction of silicon uptake in response to damage appear to be complex (Kindomihou, Sinsin
& Meerts 2006; Soininen et al. 2013; Quigley & Anderson 2014).
Impacts of silicon defences on herbivores vary with herbivore type
Ecological studies with invertebrates feeding on natural grasses have demonstrated strong
negative effects of plant silicon uptake on rates of herbivory and larval growth rates in a
range of species across various feeding guilds (Massey, Ennos & Hartley 2006; Massey,
Ennos & Hartley 2007b; Massey & Hartley 2009). However, to date many studies with
invertebrates have been in crop species (e.g. Goussain, Prado & Moraes 2005; Kvedaras &
Keeping 2007; Kvedaras et al. 2007; Kvedaras et al. 2009; Griffin, Hogan & Schmidt 2015)
and some have involved measuring effects when silicon has been sprayed on the plant
surface, rather than being taken up and deposited naturally by the plant, which is likely to
impact on herbivore responses (Moraes et al. 2004; Eswaran & Manivannan 2007).
Ecological studies on the impacts of silicon on herbivores below-ground are particularly
lacking. In one of the very few studies on this topic, silicon addition had no effect on root
herbivores (masked chafer grubs), despite causing an increase in both root and leaf silicon
content (Redmond & Potter 2006).
17
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
Similarly, only a relatively small number of studies have investigated the impacts of silicon
defences on the food preferences and performance of mammals, but there is some evidence
emerging which suggests silicon has a greater impact on the feeding behaviour of smaller
herbivores, compared to larger species. For example, laboratory studies with captive animals
have convincingly demonstrated that field voles, prairie voles (M. ochrogaster) and rabbits
(Oryctolagus cuniculus) consistently reduce their consumption of grass species containing
high concentrations of silicon (Gali-Muhtasib, Smith & Higgins 1992; Massey & Hartley
2006; Cotterill et al. 2007). Furthermore, field voles fed diets containing higher
concentrations of silicon exhibited slower growth rates and higher mortality under controlled
conditions (Massey & Hartley 2006; Huitu et al. 2014). In contrast, Massey et al. (2009)
found that sheep were less impacted by silicon defences than were smaller herbivores,
although more studies on larger grazers are required to confirm the consistency of this
pattern.
One possible reason for observed differences in effects of silicon in grasses on insects and
larger mammalian herbivores may be attributed to the differential impacts of the wearing of
teeth and mouthparts (reviewed by Strömberg, Di Stilio & Song 2016). Silicon phytoliths
have been clearly shown to cause significant and irreversible mandibular wear in the
lepidopteran Spodoptera (Massey & Hartley 2009; Reynolds, Keeping & Meyer 2009); the
extent of wear correlated with a reduction in digestive efficiency of the caterpillars,
suggesting that such wear could contribute to diet selection and the impact of silicon on
herbivore growth rates (Massey & Hartley 2009). In addition, the extent and nature of
deposition of silicon at the leaf surface has been shown to influence the abrasiveness of
natural grass species and hence potentially their vulnerability to herbivores (Hartley et al.
18
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
2015). In contrast, recent studies have demonstrated that silicon phytoliths in many grass
species are softer than tooth enamel of mammal groups including ungulates, macropods and
primates (Sanson, Kerr & Gross 2007; Rabenold & Pearson 2011; Erickson 2014; Lucas et
al. 2014; Rivals et al. 2014). However, there is evidence that some grass species contain
phytoliths that are harder than tooth enamel (Erickson 2014), although whether these are
selected by herbivores is unclear. Furthermore, Calandra et al. (2016) found effects of silicon
on microwear patterns in the teeth of voles and have proposed this as a mechanism by which
silicon may contribute to population crashes. Hummel et al. (2011), provide a compelling
argument for a role of silicon in the evolution of high-crowned teeth, showing a strong
positive correlation between faecal silicon levels and hypsodonty across a range of large
African herbivores with differing diets and digestive systems. McArthur (2014) points out
that teeth and chewing are an often neglected, but crucial component of understanding
herbivore diet selection, especially given the importance of food processing time on
digestion. In support of this idea, high silicon levels have been shown to reduce the bite rate
of sheep (Massey et al. 2009), with impacts on processing time and digestive efficiency likely
to explain why the sheep preferred to feed on grasses low in silicon.
It has been suggested that while phytoliths may not wear down mammalian teeth, they may
reduce animals’ access to cell contents by preventing cell walls being broken apart (Massey
& Hartley 2006). Consequently, variation in age, body size and digestive physiology may
play a role in determining differential effects of silicon. Variation in bite size and offtake rate
among different size classes of herbivores may impact the induction of silicon defences,
whilst the greater amount of biomass ingested by large herbivores could potentially dilute the
potency of silicon defence. Negative relationships between herbivore body size and diet
quality as a result of increased digestive efficiency have been well described (Bell 1970;
19
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
Jarman 1974), although a recent paper by Steuer et al. (2014) challenges this paradigm.
Research to date has generally focussed on the positive aspects of animal nutrition, but an
understanding of the interactive role of plant defences on the digestibility of plants for
different size and age classes of herbivores is missing. Silicon defences in grasses are an
excellent system to test such nutritional hypotheses.
Most grazers have developed the ability to digest a lot of fibre in grasses, but not silicon.
Thus, it may act as an effective bulking agent and prevent fibre (structural carbohydrates),
and ultimately, dry matter digestibility (Shewmaker et al. 1989). As epidermal silicon can
prevent enzyme-aided infiltration by fungal hyphae (Fauteux et al. 2006), it seems likely that
it can protect some of the fibre fractions from degradation by cellulases. Watling et al. (2011)
found that carbon occluded in phytoliths includes cellulose, lignin and carboxylic acids,
which suggests that there could be some chemical interaction between these fractions. In
addition, silicon is likely to impact on nitrogen absorption by preventing the leaf cell walls
being broken apart (Massey & Hartley 2006; Hunt et al. 2008), which is presumably one way
silicon reduces growth rates and fecundity of voles and insects. The impact is predicted to be
more marked in small, hindgut-fermenting herbivores, such as voles which are more likely to
be N limited, compared to the larger ruminants which can utilise endogenous sources of N, or
lagomorphs which practice caecotrophy to avoid N limitation. Nevertheless, silicon has been
shown to inhibit microbial digestion in ruminants (Harbers, Raiten & Paulsen 1981), so
further studies are required to validate this hypothesis. Wieczorek et al. (2015a) elucidated
the physiological mechanisms underpinning the negative effects of an abrasive plant diet on
the performance of root voles (M. oeconomus). Voles fed a diet of sedges containing silicon
and high concentrations of fibre had reduced absorptive efficiency in the small intestine, with
shorter villi and more mucus cells, compared to controls. Consequently, these animals had
20
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
reduced body mass and lower resting metabolic rate, which they suggested was because voles
were unable to increase food intake sufficiently to compensate for the impacts of
abrasiveness on the gut. Further studies on the physiological impact of silicon abrasiveness
on vertebrate guts would be intriguing.
The impacts of silicon on herbivore growth rates and reproduction are predicted to be more
significant for herbivores that exhibit population cycles, such as voles (Reynolds et al. 2012),
since the negative feedback from delayed density-dependence of silicon induction in relation
to herbivore density provides a nutritional mechanism to explain population regulation
(Massey & Hartley 2006; Massey et al. 2008; Wieczorek et al. 2015b). Conversely, feedback
between herbivore population density and grazing pressure means that cyclic herbivore
species are more likely to drive patterns of silicon induction, compared to non-cyclic
herbivores (Wieczorek et al. 2015b). Theoretical models have provided support for this
hypothesis, indicating that a threshold level of herbivore damage is required to initiate
sufficient silicon induction to elicit population cycles (Reynolds et al. 2012). Recently,
Wieczorek et al. (2015b) demonstrated that grazing by voles at a spatial scale relevant to their
home ranges resulted in significant induction of silicon defences in sedges in Poland, while
Massey et al. (2008) found correlations between silicon levels in D. caespitosa and M.
agrestis densities in northern England. However, as yet there have been no empirical studies
in natural grasslands at the landscape scale relevant to animal populations which
convincingly demonstrate that vole grazing pressure is sufficient to induce silicon defences to
the level required to affect herbivore population dynamics (Hartley 2015). Nevertheless, the
work in Polish and English grassland systems, including the advances in understanding the
effects of eating high-silicon diets on animals’ digestive physiology (Wieczorek et al. 2015a),
gives some support to the hypothesis that silicon defences may drive vole population cycles.
21
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
The next step is to expand these studies to understand how local effects of silicon on vole
meta-populations drive population cycles at a landscape scale.
Landscape scale studies of wild herbivore populations
Linking plant defence to the regulation of wild herbivore populations is inherently difficult
(Bazely et al. 1997; Foley, Iason & Makkar 2007; DeGabriel et al. 2014). Two studies have
successfully demonstrated relationships between N availability and reproductive success in
mammal populations mediated by constitutive tannin concentrations (DeGabriel et al. 2009;
McArt et al. 2009), but no such relationships have as yet been demonstrated for induced
defence systems. Attempts to link silicon defences to mammal population cycles are
hampered by the complexity of the diets of wild herbivores in natural grasslands, which may
result in insufficient grazing pressure on a single plant species to induce silicon to levels
comparable to those producing anti-herbivore effects in no-choice laboratory studies.
Secondly, spatial variation in silicon concentrations as a result of the biotic and abiotic
factors described above means that averaged values for a site may under- or over-estimate the
extreme values that animals ingest within their home ranges. Finally, at certain time points,
e.g. during the “crash phase” of a cycle, natural grazing intensity may be insufficient to elicit
high levels of silicon induction. These effects are evident from Figure 1a as, despite the
similarities in patterns of silicon induction between the laboratory and field studies, the
absolute concentrations of silicon in the plants grown in the glasshouse were significantly
higher than the plants from the field. Given the complexities of the field environment, to
reveal relationships between induction of silicon defences and herbivore population dynamics
in natural grasslands we need to first obtain quantitative data on the intensity of grazing on
individual grass species in order to have confidence that herbivores are eating silicon-
accumulating plants. We also need to design sampling strategies with sufficient numbers of
22
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
samples collected across an appropriate spatial scale to capture the variation in silicon
concentrations in field environments. We need to select places and times where herbivore
densities are high enough to elicit sufficient grazing pressure to exceed the threshold required
to cause induction of silicon defences. Finally, we need to be aware of abiotic factors that
may impact on silicon induction, as described above, and use this information to inform our
selection of sites and the timing of our experimental manipulations and sample collection.
Conclusion
Much is now known about silicon-based defences in grasses and their impact on herbivores
(Figure 2), though it is also clear that silicon defences in natural grasses exhibit enormous
variability, both within and between species. Induction of silicon defences is affected by
abiotic factors such as soil silicon availability, by variation in biological process such as
transpiration rates, and by plant genotype, as well as by the amount and type of damage a
plant receives (Figure 2). However, much of this current understanding has been derived
from studies in the laboratory and glasshouse, which is in large part due to difficulties
inherent in field studies, where multiple, interacting factors may simultaneously impact on
the uptake and use of silicon for defence. Although relationships between silicon
concentration and animal feeding preferences and performance can be demonstrated in the
laboratory (Massey & Hartley 2006; Massey, Ennos & Hartley 2007b), as we increase spatial
scale the effects of grazing on silica induction become harder to demonstrate, particularly at a
landscape scale (Soininen et al. 2013; Huitu et al. 2014; Wieczorek et al. 2015b). Only a
handful of large-scale studies have been conducted so far, but encouragingly, the patterns of
induction, in terms of threshold damage levels required, time for it to occur and its magnitude
seem similar in the glasshouse and in enclosures (Figure 1). Crucially though, we still lack a
landscape scale demonstration of the impact of herbivores on silicon induction and vice
23
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
versa. This does not indicate that silicon defences do not have any functional relevance in real
ecosystems. Rather, ecologists need to overcome the difficulties inherent in observing effects
at landscape scales where there is a need to tease apart the confounding factors that could
impact silicon induction and its effect on herbivores. There are a number of other such key
knowledge gaps which currently prevent us having a full understanding of the ecological role
of silicon-based defences against herbivores. We highlight some of them in Figure 2 and
suggest them as potential future research areas to provide novel insights into the mechanisms
by which silicon can underpin plant-herbivore interactions in grasses.
Acknowledgements
We thank Julia Cooke, Ben Moore and anonymous referees for comments on earlier drafts.
Xavier Lambin, Fergus Massey, Stefan Reidinger and Elizabeth Herridge contributed to the
unpublished study described in Figure 1, which was funded by NERC grants to SEH and XL
((NE/F003137/1 and NE/F003994/1).
24
570
571
572
573
574
575
576
577
578
579
580
581
582
583
References
Alborn, H.T., Turlings, T.C.J., Jones, T.H., Stenhagen, G., Loughrin, J.H. & Tumlinson, J.H.
(1997) An elicitor of plant volatiles from beet armyworm oral secretion. Science, 276,
945-949.
Banuelos, M.J. & Obeso, J.R. (2000) Effect of grazing history, experimental defoliation, and
genotype on patterns of silicification in Agrostis tenuis Sibth. Ecoscience, 7, 45-50.
Bazely, D.R., Vicari, M., Emmerich, S., Filip, L., Lin, D. & Inman, A. (1997) Interactions
between herbivores and endophyte-infected Festuca rubra from the Scottish islands of
St. Kilda, Benbecula and Rum. Journal of Applied Ecology, 34, 847-860.
Beckwith, R.S. & Reeve, R. (1964) Studies on soluble silica in soils. II. The release of
monosilicic acid from soils. Australian Journal of Soil Research, 2, 157-168.
Bell, R.H.V. (1970) The use of the herb layer by grazing ungulates in the Serengeti. Animal
Populations and Relations to their Food Resources (ed. A. Watson), pp. 111-124.
Blackwell, Oxford.
Bonaventure, G. (2012) Perception of insect feeding by plants. Plant Biology, 14, 872-880.
Bonaventure, G., VanDoorn, A. & Baldwin, I.T. (2011) Herbivore-associated elicitors: FAC
signaling and metabolism. Trends in Plant Science, 16, 294-299.
Brizuela, M.A., Detling, J.K. & Cid, M.S. (1986) Silicon concentration of grasses growing in
sites with different grazing histories. Ecology, 67, 1098-1101.
Calandra, I., Zub, K., Szafranska, P.A., Zalewski, A. & Merceron, G. (2016) Silicon-based
plant defences, tooth wear and voles. Journal of Experimental Biology, 219, 501-507.
Cid, M.S., Detling, J.K., Brizuela, M.A. & Whicker, A.D. (1989) Patterns in grass
silicification: response to grazing history and defoliation. Oecologia, 80, 268-271.
25
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
Cid, M.S., Detling, J.K., Whicker, A.D. & Brizuela, M.A. (1990) Silicon uptake and
distribution in Agropyron smithii as related to grazing history and defoliation. Journal
of Range Management, 43, 344-346.
Cooke, J. & Leishman, M.R. (2011) Is plant ecology more siliceous than we realise? Trends
in Plant Science, 16, 61-68.
Cotterill, J.V., Watkins, R.W., Brennon, C.B. & Cowan, D.P. (2007) Boosting silica levels in
wheat leaves reduces grazing by rabbits. Pest Management Science, 63, 247-253.
Currie, H.A. & Perry, C.C. (2007) Silica in plants: Biological, biochemical and chemical
studies. Annals of Botany, 100, 1383-1389.
Damuth, J. & Janis, C.M. (2011) On the relationship between hypsodonty and feeding
ecology in ungulate mammals, and its utility in palaeoecology. Biological Reviews,
86, 733-758.
DeGabriel, J.L., Moore, B.D., Felton, A.M., Ganzhorn, J.U., Stolter, C., Wallis, I.R.,
Johnson, C.N. & Foley, W.J. (2014) Translating nutritional ecology from the
laboratory to the field: milestones in linking plant chemistry to population regulation
in mammalian browsers. Oikos, 123, 298-308.
DeGabriel, J.L., Moore, B.D., Foley, W.J. & Johnson, C.N. (2009) The effects of plant
defensive chemistry on nutrient availability predict reproductive success in a
mammal. Ecology, 90, 711-719.
Deshmukh, R.K. & Belanger, R.R. (2016) Molecular evolution of aquaporins and silicon
influx in plants. Functional Ecology, In press.
Detling, J.K. & Painter, E.L. (1983) Defoliation responses of western wheatgrass populations
with diverse histories of prarie dog grazing. Oecologia, 57, 65-71.
26
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
Ehrlich, H., Demadis, K.D., Pokrovsky, O.S. & Koutsoukos, P.G. (2010) Modern Views on
Desilicification: Biosilica and Abiotic Silica Dissolution in Natural and Artificial
Environments. Chemical Reviews, 110, 4656-4689.
Epstein, E. (1999) Silicon. Annual Review of Plant Physiology and Plant Molecular Biology,
50, 641-664.
Ergon, T., Ergon, R., Begon, M., Telfer, S. & Lambin, X. (2011) Delayed density-dependent
onset of spring reproduction in a fluctuating population of field voles. Oikos, 120,
934-940.
Ergon, T., Lambin, X. & Stenseth, N.C. (2001) Life-history traits of voles in a fluctuating
population respond to the immediate environment. Nature, 411, 1043-1045.
Erickson, K.L. (2014) Prairie grass phytolith hardness and the evolution of ungulate
hypsodonty. Historical Biology, 26, 737-744.
Eswaran, A. & Manivannan, K. (2007) Effect of foliar application of lignite fly ash on the
management of papaya leaf curl disease. Acta Horticulturae, 740, 271-275.
Faisal, S., Callis, K.L., Slot, M. & Kitajima, K. (2012) Transpiration-dependent passive silica
accumulation in cucumber (Cucumis sativus) under varying soil silicon availability.
Botany-Botanique, 90, 1058-1064.
Farmer, E. (2014) Leaf Defence. Oxford University Press, Oxford, U.K.
Fauteux, F., Chain, F., Belzile, F., Menzies, J.G. & Belanger, R.R. (2006) The protective role
of silicon in the Arabidopsis-powdery mildew pathosystem. Proceedings of the
National Academy of Sciences of the United States of America, 103, 17554-17559.
Fenner, M., Lee, W.G. & Duncan, S.J. (1993) Chemical features of Chionochloa species in
relation to grazing by ruminants in South Island, New Zealand. New Zealand Journal
of Ecology, 17, 35-40.
27
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
Foley, W., Iason, G. & Makkar, H. (2007) Transdisciplinary studies of plant secondary
metabolites: Lessons from ecology for animal science and vice versa. Proceedings of
the 7th International Symposium on the Nutrition of Herbivores (eds Q.X. Meng, J.X.
Liu & W.Y. Zhu). Beijing.
Gali-Muhtasib, H.U., Smith, C.C. & Higgins, J.J. (1992) The effect of silica in grasses on the
feeding behavior of the prairie vole Microtus ochrogaster. Ecology, 73, 1724-1729.
Garbuzov, M., Reidinger, S. & Hartley, S.E. (2011) Interactive effects of plant-available soil
silicon and herbivory on competition between two grass species. Annals of Botany,
108, 1355-1363.
Georgiadis, N.J. & McNaughton, S.J. (1990) Elemental and fiber contents of savanna grasses:
variation with grazing, soil type, season and species. Journal of Applied Ecology, 27,
623-634.
Gomes, F.B., de Moraes, J.C., dos Santos, C.D. & Goussain, M.M. (2005) Resistance
induction in wheat plants by silicon and aphids. Scientia Agricola, 62, 547-551.
Goussain, M.M., Prado, E. & Moraes, J.C. (2005) Effect of silicon applied to wheat plants on
the biology and probing behaviour of the greenbug Schizaphis graminum (Rond.)
(Hemiptera : Aphididae). Neotropical Entomology, 34, 807-813.
Griffin, M., Hogan, B. & Schmidt, O. (2015) Silicon reduces slug feeding on wheat
seedlings. Journal of Pest Science, 88, 17-24.
Harbers, L.H., Raiten, D.J. & Paulsen, G.M. (1981) The role of plant epidermal silica as a
structural inhibitor of rumen microbial digestion in steers. Nutrition Reports
International, 24, 1057-1066.
Harbers, L.H. & Thouvenelle, M.L. (1980) Digestion of corn and sorghum silage observed by
scanning electron microscopy. Journal of Animal Science, 50, 514-526.
28
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
Hartley, S.E. (2015) Round and round in cycles? Silicon-based plant defences and vole
population dynamics. Functional Ecology, 29, 151-153.
Hartley, S.E., Fitt, R.N., McLamon, E.L. & Wade, R.N. (2015) Defending the leaf surface:
intra- and inter-specific differences in silicon deposition in grasses in response to
damage and silicon supply. Frontiers in Plant Science, 6, 35.
Hartley, S.E. & Lawton, J.H. (1987) Effects of different types of damage on the chemistry of
birch foliage, and the responses of birch feeding insects. Oecologia, 74, 432-437.
Hartley, S.E. & Lawton, J.H. (1991) Biochemical aspects and significance of the rapidly
induced accumulation of phenolics in birch foliage. Phytochemical Induction by
Herbivores (eds D.W. Tallamy & M.J. Raupp), pp. 105-132. Wiley, New York.
Haukioja, E. & Neuvonen, S. (1985) Induced long-term resistance of birch foliage against
defoliators - defensive or incidental? Ecology, 66, 1303-1308.
Heil, M. & Ton, J. (2008) Long-distance signalling in plant defence. Trends in Plant Science,
13, 264-272.
Hodson, M.J., White, P.J., Mead, A. & Broadley, M.R. (2005) Phylogenetic variation in the
silicon composition of plants. Annals of Botany, 96, 1027-1046.
Huitu, O., Forbes, K.M., Helander, M., Julkunen-Tiitto, R., Lambin, X., Saikkonen, K.,
Stuart, P., Sulkama, S. & Hartley, S. (2014) Silicon, endophytes and secondary
metabolites as grass defenses against mammalian herbivores. Frontiers in Plant
Science, 5, 478.
Hummel, J., Findeisen, E., Suedekum, K.-H., Ruf, I., Kaiser, T.M., Bucher, M., Clauss, M. &
Codron, D. (2011) Another one bites the dust: faecal silica levels in large herbivores
correlate with high-crowned teeth. Proceedings of the Royal Society B-Biological
Sciences, 278, 1742-1747.
29
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
Hunt, J.W., Dean, A.P., Webster, R.E., Johnson, G.N. & Ennos, A.R. (2008) A novel
mechanism by which silica defends grasses against herbivory. Annals of Botany, 102,
653-656.
Jarman, P.J. (1974) Social organization of antelope in relation to their ecology. Behaviour,
48, 215-267.
Karban, R. & Baldwin, I.T. (1997) Induced Responses to Herbivory. The University of
Chicago Press, Chicago.
Katz, O. (2015) Silica phytoliths in angiosperms: phylogeny and early evolutionary history.
New Phytologist, 208, 642-646.
Keeping, M.G., Meyer, J.H. & Sewpersad, C. (2013) Soil silicon amendments increase
resistance of sugarcane to stalk borer Eldana saccharina Walker (Lepidoptera:
Pyralidae) under field conditions. Plant and Soil, 363, 297-318.
Keeping, M.G., Miles, N. & Sewpersad, C. (2014) Silicon reduces impact of plant nitrogen in
promoting stalk borer (Eldana saccharina) but not sugarcane thrips (Fulmekiola
serrata) infestations in sugarcane. Frontiers in Plant Science, 5, 289.
Keeping, M.G. & Reynolds, O.L. (2009) Silicon in agriculture: new insights, new
significance and growing application. Annals of Applied Biology, 155, 153-154.
Kindomihou, V., Sinsin, B. & Meerts, P. (2006) Effect of defoliation on silica accumulation
in five tropical fodder grass species in Benin. Belgian Journal of Botany, 139, 87-102.
Korth, K.L. & Dixon, R.A. (1997) Evidence for chewing insect-specific molecular events
distinct from a general wound response in leaves. Plant Physiology, 115, 1299-1305.
Kvedaras, O.L., Byrne, M.J., Coombes, N.E. & Keeping, M.G. (2009) Influence of plant
silicon and sugarcane cultivar on mandibular wear in the stalk borer Eldana
saccharina. Agricultural and Forest Entomology, 11, 301-306.
30
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
Kvedaras, O.L. & Keeping, M.G. (2007) Silicon impedes stalk penetration by the borer
Eldana saccharina in sugarcane. Entomologia Experimentalis Et Applicata, 125, 103-
110.
Kvedaras, O.L., Keeping, M.G., Goebel, F.R. & Byrne, M.J. (2007) Larval performance of
the pyralid borer Eldana saccharina Walker and stalk damage in sugarcane: Influence
of plant silicon, cultivar and feeding site. International Journal of Pest Management,
53, 183-194.
Lindroth, R.L. & Batzli, G.O. (1986) Inducible plant chemical defences - A cause of vole
population cycles. Journal of Animal Ecology, 55, 431-449.
Lucas, P.W., van Casteren, A., Al-Fadhalah, K., Almusallam, A.S., Henry, A.G., Michael, S.,
Watzke, J., Reed, D.A., Diekwisch, T.G.H., Strait, D.S. & Atkins, A.G. (2014) The
role of dust, grit and phytoliths in tooth wear. Annales Zoologici Fennici, 51, 143-152.
Ma, J.F. (2004) Role of silicon in enhancing the resistance of plants to biotic and abiotic
stresses. Soil Science and Plant Nutrition, 50, 11-18.
Ma, J.F. (2009) Silicon uptake and translocation in plants. The Proceedings of the
International Plant Nutrition Colloquium XVI. Department of Plant Sciences, UC
Davis, UC Davis.
Ma, J.F., Tamai, K., Yamaji, N., Mitani, N., Konishi, S., Katsuhara, M., Ishiguro, M.,
Murata, Y. & Yano, M. (2006) A silicon transporter in rice. Nature, 440, 688-691.
Ma, J.F. & Yamaji, N. (2006) Silicon uptake and accumulation in higher plants. Trends in
Plant Science, 11, 392-397.
Ma, J.F. & Yamaji, N. (2015) A cooperative system of silicon transport in plants. Trends in
Plant Science, 20, 435-442.
31
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
Massey, F.P., Ennos, A.R. & Hartley, S.E. (2006) Silica in grasses as a defence against insect
herbivores: contrasting effects on folivores and a phloem feeder. Journal of Animal
Ecology, 75, 595-603.
Massey, F.P., Ennos, A.R. & Hartley, S.E. (2007a) Grasses and the resource availability
hypothesis: the importance of silica-based defences. Journal of Ecology, 95, 414-424.
Massey, F.P., Ennos, A.R. & Hartley, S.E. (2007b) Herbivore specific induction of silica-
based plant defences. Oecologia, 152, 677-683.
Massey, F.P. & Hartley, S.E. (2006) Experimental demonstration of the antiherbivore effects
of silica in grasses: impacts on foliage digestibility and vole growth rates.
Proceedings of the Royal Society B-Biological Sciences, 273, 2299-2304.
Massey, F.P. & Hartley, S.E. (2009) Physical defences wear you down: progressive and
irreversible impacts of silica on insect herbivores. Journal of Animal Ecology, 78,
281-291.
Massey, F.P., Massey, K., Ennos, A.R. & Hartley, S.E. (2009) Impacts of silica-based
defences in grasses on the feeding preferences of sheep. Basic and Applied Ecology,
10, 622-630.
Massey, F.P., Smith, M.J., Lambin, X. & Hartley, S.E. (2008) Are silica defences in grasses
driving vole population cycles? Biology letters, 4, 419-422.
McArt, S.H., Spalinger, D.E., Collins, W.B., Schoen, E.R., Stevenson, T. & Bucho, M.
(2009) Summer dietary nitrogen availability as a potential bottom-up constraint on
moose in south-central Alaska. Ecology, 90, 1400-1411.
McArthur, C. (2014) Do we ditch digestive physiology in explaining the classic relationship
between herbivore body size diet and diet quality? Functional Ecology, 28, 1059-
1060.
32
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
McColloch, J.W. & Salmon, S.C. (1923) The resistance of wheat to the Hessian Fly - a
progress report. Journal of Economic Entomology, 16, 293-298.
McNaughton, S.J. & Tarrants, J.L. (1983) Grass leaf silicification - natural selection for an
inducible defense against herbivores. Proceedings of the National Academy of
Sciences of the United States of America-Biological Sciences, 80, 790-791.
McNaughton, S.J., Tarrants, J.L., McNaughton, M.M. & Davis, R.H. (1985) Silica as a
defense against herbivory and a growth promotor in African grasses. Ecology, 66,
528-535.
Moraes, J.C., Goussain, M.M., Basagli, M.A.B., Carvalho, G.A., Ecole, C.C. & Sampaio,
M.V. (2004) Silicon influence on the tritrophic interaction: Wheat plants, the
greenbug Schizaphis graminum (Rondani) (Hemiptera : Aphididae), and its natural
enemies, Chrysoperla externa (Hagen) (Neuroptera : Chrysopidae) and Aphidius
colemani viereck (Hymenoptera : Aphidiidae). Neotropical Entomology, 33, 619-624.
Piperno, D.R. (2006) Pytoliths: A comprehensive Guide for Archaeologists and
Palaeoecologists. AltaMira Press, Oxford.
Ponnaiya, B. (1951) Studies on the genus Sorghum. II The cause of resistance in sorghum to
the insect pest Atherigona indica M. Madras University Journal, 21, 203-217.
Quigley, K.M. & Anderson, T.M. (2014) Leaf silica concentration in Serengeti grasses
increases with watering but not clipping: insights from a common garden study and
literature review. Frontiers in Plant Science, 5, 568.
Rabenold, D. & Pearson, O.M. (2011) Abrasive, silica phytoliths and the evolution of thick
molar enamel in primates, with implications for the diet of Paranthropus boisei. Plos
One, 6, e28379.
Raven, J.A. (1983) The transport and function of silicon in plants. Biological Reviews of the
Cambridge Philosophical Society, 58, 179-207.
33
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
Redmond, C.T. & Potter, D.A. (2006) Silicon fertilization does not enhance creeping
bentgrass resistance to cutworms and white grubs. Applied Turfgrass Science
[Online], 6, 1-7.
Reidinger, S., Ramsey, M.H. & Hartley, S.E. (2012) Rapid and accurate analyses of silicon
and phosphorus in plants using a portable X-ray fluorescence spectrometer. New
Phytologist, 195, 699-706.
Reymond, P., Weber, H., Damond, M. & Farmer, E.E. (2000) Differential gene expression in
response to mechanical wounding and insect feeding in Arabidopsis. Plant Cell, 12,
707-719.
Reynolds, J.J.H., Lambin, X., Massey, F.P., Reidinger, S., Sherratt, J.A., Smith, M.J., White,
A. & Hartley, S.E. (2012) Delayed induced silica defences in grasses and their
potential for destabilising herbivore population dynamics. Oecologia, 170, 445-456.
Reynolds, O.L., Keeping, M.G. & Meyer, J.H. (2009) Silicon-augmented resistance of plants
to herbivorous insects: a review. Annals of Applied Biology, 155, 171-186.
Rivals, F., Takatsuki, S., Maria Albert, R. & Macia, L. (2014) Bamboo feeding and tooth
wear of three sika deer (Cervus nippon) populations from northern Japan. Journal of
Mammalogy, 95, 1043-1053.
Sangster, A.G., Hodson, M.J. & Tubb, H.J. (2001) Silicon deposition in higher plants. Silicon
in Agriculture (eds L.E. Datnoff, G.H. Snyder & G.H. Korndorfer), pp. 85-113.
Elsevier Science, Amsterdam.
Sanson, G.D., Kerr, S.A. & Gross, K.A. (2007) Do silica phytoliths really wear mammalian
teeth? Journal of Archaeological Science, 34, 526-531.
Sasamoto, K. (1953) Studies on the relation between insect pests and silica content in rice
plant (II). On the injury of the second generation larvae of rice stem borer. Oyo
Kontyu, 9, 108-110.
34
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
Seastedt, T.R., Ramundo, R.A. & Hayes, D.C. (1989) Silica, nitrogen and phosphorus
dynamics of tallgrass prairie. 11th North American Prairie Conference, pp. 205-210.
University of Nebraska Press.
Shewmaker, G.E., Mayland, H.F., Rosenau, R.C. & Asay, K.H. (1989) Silicon in C3 grasses -
effects on forage quality and sheep preference. Journal of Range Management, 42,
122-127.
Smith, M.J., White, A., Lambin, X., Sherratt, J.A. & Begon, M. (2006) Delayed density-
dependent season length alone can lead to rodent population cycles. American
Naturalist, 167, 695-704.
Soininen, E.M., Brathen, K.A., Jusdado, J.G.H., Reidinger, S. & Hartley, S.E. (2013) More
than herbivory: levels of silica-based defences in grasses vary with plant species,
genotype and location. Oikos, 122, 30-41.
Stam, J.M., Kroes, A., Li, Y., Gols, R., van Loon, J.J.A., Poelman, E.H. & Dicke, M. (2014)
Plant interactions with multiple insect herbivores: From community to genes. Annual
Review of Plant Biology, 65, 689-713.
Steuer, P., Suedekum, K.-H., Tuetken, T., Mueller, D.W.H., Kaandorp, J., Bucher, M.,
Clauss, M. & Hummel, J. (2014) Does body mass convey a digestive advantage for
large herbivores? Functional Ecology, 28, 1127-1134.
Strömberg, C., Di Stilio, V. & Song, Z. (2016) Functions of phytoliths in vascular plants: An
evolutionary perspective. Functional Ecology, In press.
Tanentzap, A.J., Vicari, M. & Bazely, D.R. (2014) Ungulate saliva inhibits a grass-endophyte
mutualism. Biology letters, 10.
Tian, D., Peiffer, M., Shoemaker, E., Tooker, J., Haubruge, E., Francis, F., Luthe, D.S. &
Felton, G.W. (2012) Salivary glucose oxidase from caterpillars mediates the induction
of rapid and delayed-induced defenses in the tomato plant. Plos One, 7, e36168.
35
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
Valkama, E., Koricheva, J., Ossipov, V., Ossipova, S., Haukioja, E. & Pihlaja, K. (2005)
Delayed induced responses of birch glandular trichomes and leaf surface lipophilic
compounds to mechanical defoliation and simulated winter browsing. Oecologia, 146,
385-393.
Vicari, M. & Bazely, D.R. (1993) Do grasses fight back? The case for antiherbivore
defences. Trends in Ecology & Evolution, 8, 137-141.
Walters, D. (2010) Plant Defense: Warding off Attack by Pathogens, Herbivores and
Parasitic Plants. Wiley-Blackwell.
Watling, K.M., Parr, J.F., Rintoul, L., Brown, C.L. & Sullivan, L.A. (2011) Raman, infrared
and XPS study of bamboo phytoliths after chemical digestion. Spectrochimica Acta
Part A-Molecular and Biomolecular Spectroscopy, 80, 106-111.
Wieczorek, M., Szafranska, P.A., Labecka, A.M., Lazaro, J. & Konarzewski, M. (2015a)
Effect of the abrasive properties of sedges on the intestinal absorptive surface and
resting metabolic rate of root voles. Journal of Experimental Biology, 218, 309-315.
Wieczorek, M., Zub, K., Szafranska, P.A., Ksiazek, A. & Konarzewski, M. (2015b) Plant-
herbivore interactions: silicon concentration in tussock sedges and population
dynamics of root voles. Functional Ecology, 29, 187-194.
Yamaji, N., Sakurai, G., Mitani-Ueno, N. & Ma, J.F. (2015) Orchestration of three
transporters and distinct vascular structures in node for intervascular transfer of
silicon in rice. Proceedings of the National Academy of Sciences of the United States
of America, 112, 11401-11406.
Ye, M., Song, Y., Long, J., Wang, R., Baerson, S.R., Pan, Z., Zhu-Salzman, K., Xie, J., Cai,
K., Luo, S. & Zeng, R. (2013) Priming of jasmonate-mediated antiherbivore defense
responses in rice by silicon. Proceedings of the National Academy of Sciences of the
United States of America, 110, E3631-E3639.
36
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
Figure legends
Figure 1
Induction of silica defences in Deschampsia caespitosa exposed to high (~20% of leaves
damaged) and low (~5% of leaves damaged) levels of grazing by field voles (Microtus
agrestis). Solid lines denote high grazing intensity and broken lines denote low grazing
intensity. (a) Comparison of silica induction in D. caespitosa grown in glasshouse and grazed
in the laboratory (reproduced from Reynolds et al. 2012) and under field conditions in open
grazing enclosures in northern England from May-November 2009 with a grazing intensity of
300 voles/ha and 50 voles/ha (J. DeGabriel, S. Hartley, F. Massey, S. Reidinger and X.
Lambin, unpublished data). (b) Seasonal variation in silica concentrations in D. caespitosa in
field grazing enclosures under high (300 voles/ha) and low (50 voles/ha) grazing treatments
from March 2009-April 2010. Error bars represent standard error.
Figure 2
A summary of research needs for silicon-mediated ecological interactions between
plants and herbivores. Green boxes summarise established knowledge, whilst the pink
boxes suggest key knowledge gaps and potential research questions for future work, as
depicted by the graphics in circles.
37
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
0 1 2 3 4 5 60
1
2
3
4
5
6
7
8
Month
Mea
n sil
ica
conc
entr
ation
(%DM
)
Spring Summer Autumn Winter Spring0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Season
Mea
n sil
ica
conc
entr
ation
(% D
M)
38
892
893
894